Systems and methods for resonator fiber optic gyroscope intensity modulation control

ABSTRACT

Systems and methods for improved resonator fiber optic gyroscope intensity modulation control are provided. In one embodiment, a resonant fiber optic gyroscope (RFOG) having a residual intensity modulation (RIM) controller comprises: an intensity modulator optically coupled to receive a light beam from a laser source modulated at a resonance detection modulation frequency; an optical tap device optically coupled to the intensity modulator; and a feedback servo coupled to the optical tap device and the intensity modulator, the demodulating feedback servo generating a sinusoidal feedback signal to the intensity modulator. The feedback servo adjusts an amplitude and phase of the sinusoidal feedback signal provided to intensity modulator based on a residual intensity modulation detected by the demodulating feedback servo.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/165,697, titles “LIGHT INTENSITY MODULATION CONTROL” filed Apr.1, 2009, of which is hereby incorporated by reference.

BACKGROUND

Resonator fiber optic gyros (RFOGs) have tremendous potential formeeting the needs of many navigation and attitude control markets. Onemajor source of rotation error that could limit the realized RFOGperformance is intensity modulation that is generated when performingfrequency or phase modulation of the light that is used for probing theresonator resonance frequency and determining rotation. Within the artavailable today, to achieve performance potentials the intensitymodulation from existing frequency or phase modulators needs to bereduced by as much as 60 dB at the resonance detection modulationfrequency. A servo that directly controls the intensity of the light viaan intensity modulator can be used, however a very high speed intensitymodulator would need to be employed. Integrated optics intensitymodulators made from Lithium Niobate waveguides provide very high speedintensity modulation, but tend to have significant optical loss, arerelatively large and costly. There are other types of intensitymodulators that have significantly lower optical loss, are small andinexpensive, but do not provide the speed necessary to employ prior artintensity servos.

For the reasons stated above and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the specification, there is a need in the art for improvedsystems and methods for resonator fiber optic gyroscope intensitymodulation control.

SUMMARY

The embodiments of the present invention provide methods and systems forresonator fiber optic gyroscope intensity modulation control and will beunderstood by reading and studying the following specification.

In one embodiment, a resonant fiber optic gyroscope (RFOG) having aresidual intensity modulation (RIM) controller comprises: an intensitymodulator optically coupled to receive a light beam from a laser sourcemodulated at a resonance detection modulation frequency; an optical tapdevice optically coupled to the intensity modulator; and a feedbackservo coupled to the optical tap device and the intensity modulator, thedemodulating feedback servo generating a sinusoidal feedback signal tothe intensity modulator. The feedback servo adjusts an amplitude andphase of the sinusoidal feedback signal provided to intensity modulatorbased on a residual intensity modulation detected by the demodulatingfeedback servo.

DRAWINGS

Embodiments of the present invention can be more easily understood andfurther advantages and uses thereof more readily apparent, whenconsidered in view of the description of the preferred embodiments andthe following figures in which:

FIG. 1 is a block diagram illustrating a feedback loop circuit having ademodulating feedback servo of one embodiment of the present invention;

FIG. 2 is a block diagram illustrating a demodulating feedback servo ofone embodiment of the present invention;

FIG. 3 is a block diagram of a resonator fiber-optic gyroscope (RFOG),including a residual intensity modulation (RIM) control loop accordingto one embodiment of the invention; and

FIG. 4 is a flow chart illustrating a method of one embodiment of thepresent invention.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize features relevant to thepresent invention. Reference characters denote like elements throughoutfigures and text.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of specific illustrative embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that logical,mechanical and electrical changes may be made without departing from thescope of the present invention. The following detailed description is,therefore, not to be taken in a limiting sense.

In a resonator fiber optic gyroscope (RFOG), phase and frequencymodulation is used to detect the center resonance frequency of the RFOGresonator. Intensity modulation is an undesired byproduct produced fromperforming either phase or frequency modulation of the RFOG light beams.The presence of intensity modulation on the light beam, when it occursat the Resonance detection modulation frequency, will result in thegeneration of rate bias errors in the gyroscope. The purpose of aResidual Intensity Modulation (or RIM) servo in the RFOG is to reduceintensity modulation down to a level where it no loner impedes gyroperformance.

Rather than taking an optical signal that is tapped off of the RFOG mainlight beam and directly feeding back to an intensity modulator,embodiments of the present invention provide methods and systems for aRIM servo that controls out intensity modulations at the modulationfrequency. Embodiments of the present invention provide a feedbacksignal in the form of a sine wave at the Resonance detection modulationfrequency. This sine wave controls both the amplitude and the phase ofthe intensity modulator for the purpose of removing the residualintensity modulation from the light beam.

FIG. 1 is block diagram illustrating a feedback loop circuit 100 for aresonator fiber optic gyroscope (RFOG) of one embodiment of the presentinvention. Feedback loop circuit 100 comprises an intensity modulator110, an optical tap device 120 (such as an optical monitor preamp, forexample) and a demodulating feedback servo 130 of one embodiment of thepresent invention. The demodulating feedback servo 130 provides the RIMservo function for feedback loop circuit 100. In operation, intensitymodulator 110 receives a light beam that includes undesired residualintensity modulation. If the residual intensity modulation were toremain in the light beam, it would be received by the resonator 140,resulting in the rate bias errors discussed above. Feedback loop circuit100 drives intensity modulator 110 to remove the undesired residualintensity modulation from the light beam, using a sinusoidal signalgenerated at the modulation frequency specified for the RFOG. Based onthe amount of residual intensity modulation that is observed at opticaltap device 120, modulating feedback servo 130 adjusts the amplitude andphase of the sinusoidal feedback signal provided to intensity modulator110 until the residual intensity modulation has been canceled out of thesignal being delivered to resonator 140. The amplitude and phaseadjustment provided via the sinusoidal feedback signal provides feedbackinformation that does not require the use of a high speed intensitymodulator for implementing intensity modulator 110.

FIG. 2 is a block diagram illustrating a demodulating feedback servo 200of one embodiment of the present invention. Demodulating feedback servo200 comprises a first control loop 210 for processing an in-phasecontrol signal (I) and a second control loop 220 for processing aquadrature phase control signal (Q). The first control loop 210 includesan in-phase demodulator 211, an accumulator 212, and a digital-to-analogconverter (DAC) 213. The second control loop 220 includes aquadrature-phase demodulator 221, an accumulator 222, and adigital-to-analog converter (DAC) 223. Demodulating feedback servo 200further comprises an analog to digital converter (ADC) 230 thatgenerates digital data samples based on the light beam received by tapdevice 120. In one embodiment, tap device 120 includes a photo-detectorthat converts the received light beam into an analog electrical signalwhich is digitized by analog to digital converter 230. In anotherembodiment, the functionality ADC 230 is integrated into tap device 120such that tap device 120 directly outputs digital data samples based onthe light beam to demodulating feedback servo 200. In one embodiment, apre-filter 226 is included to band limit the signal received by ADC 230and provide anti-alias filtering. The digitized samples are provided toboth of the two channels established by the first and second controlloops 210 and 220.

Feedback control loop 210 functions as the in-phase control loop.In-phase demodulator 210 receives the digital samples of the light beamand demodulates the digital samples at the resonance detectionmodulation frequency (which is the frequency at which it is most desiredto remove the intensity modulation). A sine-wave reference (illustratedas X-Ref) for performing this demodulation is provided by sine-wavegenerator 240. In one embodiment, the reference provided to In-phasedemodulator 210 is a square wave version of a digitally generatedsine-wave from sine-wave generator 240.

Feedback control loop 220 functions as the quadrature-phase controlloop. Quad-phase demodulator 221 also receives the digital samples ofthe light beam and demodulates the digital samples at the Resonancedetection modulation frequency using a sine-wave reference (illustratedas Y-Ref) that is 90 degrees out of phase from the X-Ref, and isprovided by sine-wave generator 242. In one embodiment, the referenceprovided to quad-phase demodulator 220 is a square wave version of adigitally generated sine-wave from sine-wave generator 242. In oneembodiment, sine-wave generators 240 and 242 are synchronized to acommon clock 244.

The output of demodulators 211 and 221 are error signals that provideinformation needed to know the amplitude and phase of the residualintensity modulation that is to be canceled out from the light beam.That is, after demodulating out the intended modulation components fromthe signal, non-zero output generated by demodulators 211 and 221 is anindication of intensity modulation in respective in-phase (I) andquadrature-phase (Q) components. The balance of demodulating feedbackservo 200 functions to provide feedback control that will strive tomaintain those two error signals at zero.

After each of the demodulators 211 and 221 are respective accumulators212 and 222 which integrate the respective I and Q error signals. Theoutput of these accumulators 212 and 222 therefore keeps changing toreflect the amount of intensity modulation remaining in the light beam.It is the output of these accumulators that control the amplitude andphase of the sinusoid used to drive the intensity modulator 110. For thefeedback control loop 210, digital-to-analog converter (DAC) 212converts the in-phase accumulated digital error data into an analogerror signal (shown as “I error Ref”). For the feedback control loop220, digital-to-analog converter (DAC) 222 converts the quad-phaseaccumulated digital error data into an analog error signal (shown as “Qerror Ref”). In one embodiment accumulators 212 and 222 may furtherprovide for truncation to condition the digital error data forconversion into analog signals by respective DACs 213 and 223. Further,in one embodiment, low pass filters 215 and 225 are employed to removeunwanted high- frequency components from the “I error Ref” and “Q errorRef” reference signals.

Amplitude and phase corrections are derived from the “I error Ref” and“Q error Ref” reference signals as follows. The addition of two sinewave signals having the same frequency, with 90 degree phase shiftbetween the two, will result in a sine wave of the same frequency. Thephase of the resulting wave, however, will depend on the relativeamplitude between the added sine waves. By adjusting the amplitudes of Iand Q reference sine waves, and then summing them together, theillustrated embodiment provides feedback control in the form of a sinewave wherein the amplitude and phase are controlled.

More specifically, in one embodiment, the “I error Ref” signal outputfrom feedback control loop 210 and the X-Ref signal from sine-wavegenerator 240 are multiplied together at Multiplying DAC 250. The outputof Multiplying DAC 250 is a version of the X-Ref signal as multiplied bythe analog “I error Ref”. If “I error Ref” is zero, indicating theexistence of no in-phase intensity modulation in the light beam, thenzero multiplied by the X-Ref signal results in a zero amplitudesinusoidal signal output from Multiplying DAC 250. For example, ifintensity modulation appears in the light beam only in quadrature, whenthe data samples are demodulated by demodulators 211 and 221, only anerror signal from the quadrature phase demodulator 221 will begenerated. No error signal will appear from in-phase demodulator 211. Tothe degree that in-phase intensity modulation does appear in the lightbeam, an error signal will accumulate in 212 resulting in a non-zero “Ierror Ref” signal for adjusting the amplitude of the sine-wave outputfrom Multiplying DAC 250.

Similarly, the “Q error Ref” signal output from feedback control loop220 and the Y-Ref signal from sine-wave generator 242 are multipliedtogether at Multiplying DAC 252. The output of Multiplying DAC 252 is aversion of the Y-Ref signal as multiplied by the analog “Q error Ref”.If “Q error Ref” is zero, indicating the existence of noquadrature-phase intensity modulation in the light beam, then zeromultiplied by the Y-Ref signal results in a zero amplitude sinusoidalsignal output from Multiplying DAC 252. For example, if intensitymodulation appears in the light beam only in in-phase, when the datasamples are demodulated by demodulators 211 and 221, only an errorsignal from the in-phase demodulator 211 will be generated. No errorsignal will appear from quadrature-phase demodulator 221. To the degreethat quadrature-phase intensity modulation does appear in the lightbeam, the error signal will accumulate in 222 resulting in a non-zero “Qerror Ref” signal for adjusting the amplitude of the sine-wave outputfrom Multiplying DAC 252.

The sine-wave outputs from Multiplying DACs 250 and 252 are summedtogether at summing amplifier 254. The resulting feedback control signalfrom summing amplifier 254 is a sine wave having a frequency equal tothe resonance detection modulation frequency, but having an amplitudeand phase that is a function of the relative amplitudes between theadded sine waves from Multiplying DACs 250 and 252. By controllingintensity modulator 110 with this sinusoidal feedback control signal,intensity modulator 110 will impart onto the light beam a modulationcomponent that cancels the residual intensity modulation byproduct.Unwanted intensity modulation is completely suppressed when bothdemodulator 211 and 221 outputs are zero.

In one embodiment, within modulating feedback servo 200, sine-wavesignal generators 240 and 242 are implemented by two direct digitalsynthesizer (DDS) chips that generate the X-Ref and Y-Ref sinusoidalsignals at a frequency equal to the gyro modulation frequency.Similarly, in one embodiment, Multiplying DACs 250 and 252 are realizedby multiplying DDS chips. Further, in one embodiment, one or more ofdemodulators 211, 221, accumulators 212, 222, clock 244, and sine-wavegenerators 240, 242 are realized via a field programmable gate array(FPGA) 260, or similar device such as, but not limited to anapplication-specific integrated circuit (ASIC). In one embodiment, oneor all of 200 are realized via a mixed-signal ASIC.

FIG. 3 provide a block diagram illustrating one embodiment of thepresent invention employing a demodulating feedback servo 332 as theresidual intensity modulation (RIM) servo within an resonatorfiber-optic gyroscope 310. In one embodiment, demodulating feedbackservo 332 adjusts the amplitude and phase of a sinusoidal feedbacksignal until the residual intensity modulation has been canceled out ofthe signal being delivered to resonator 315, as described with respectto demodulating feedback servo 130 above. In another embodiment,demodulating feedback servo 332 utilizes dual I and Q feedback controlloops as described with respect to demodulating feedback servo 200, toproduce the sinusoidal feedback signal.

FIG. 3 illustrates an RFOG 310 including two laser sources—a clockwise(CW) laser source 312 and a counterclockwise (CCW) laser source 314. Theoutputs of each of the two sources 312, 314 are coupled to the input ofa resonator 315. The output of the resonator 315 is coupled to clockwiseand counterclockwise demodulation components 316, 321 that provide afeedback signal to the clockwise and counterclockwise laser sources 312,314.

The clockwise laser source 312 includes a laser 320, a laser driver 322and a residual intensity modulation (RIM) control loop 324. The RIMcontrol loop 324 includes an intensity modulator 326, a tap coupler 328,a servo loop photo-detector 330 and demodulating feedback servo 332. Theclockwise demodulation component 316 includes a demodulationphoto-detector 317, a demodulator 318, and a demodulation processor 319.

The components of the clockwise laser source 312 are connected in aserial branch, as follows: the laser driver 322 to the laser 320 to theintensity modulator 326 to the tap coupler 328 to the resonator 315. Thecomponents of the RIM control loop 324 are connected in a series loop,as follows: the intensity modulator 326 to the tap coupler 328 to theservo loop photo-detector 330 to the demodulating feedback servo 332 andback to the intensity modulator 326. The components of the clockwisedemodulation component 316 are connected in a serial branch, as follows:the demodulation photo-detector 317 to the demodulator 318 to thedemodulation processor 319. The output of the demodulation processor 319is connected back to the laser driver 322.

The CCW laser source 314 includes a complementary set of componentsconnected in a complementary arrangement to those included in the CWlaser source 312. The CCW demodulator component 321 also includes acomplementary set of components connected in a complementary arrangementto those included in the CW demodulation component 316.

In operation, the laser driver 322 drives the laser 320 to output alight beam at a given frequency. Light from the laser 320 passes throughthe intensity modulator 326 and the tap coupler 328 before beingreceived by the resonator 315. The resonator 315 receives light fromboth the CW and CCW laser sources 312, 314, with the light beams fromthe two sources 312, 314 traveling around the resonator 315 in oppositedirections. The demodulation components 316, 321 each receive a lightbeam from the resonator 315, and by detecting a signal that isindicative of the difference in the resonance frequency and the laserfrequency, maintain each laser frequency at each corresponding resonancefrequency.

Each demodulation component 316, 321 also outputs to the laser driver322, which continuously drives the laser 320 to resonance frequency bycontinuously adjusting the frequency to keep the demodulated photodetector output signal equal to zero. This maintains each light beam atthe resonance frequency.

The measure of rotation rate is the measure of the frequency differencebetween the CW and CCW resonance frequency. Since the laser frequenciesare controlled to the resonance frequencies, the difference between thelaser frequencies is a measure of rotation rate. To measure thedifference between the laser frequencies, a small portion of light istapped off from each beam just after the lasers by tap couplers 333. Thetapped light from both beams is combined at a beam combiner 334. Thebeam combiner 334 has two outputs, each going to a photo-detector 335.The CW and CCW laser beams interfere at the two photo-detectors 335 andproduce a beat signal that is an intensity variation that has afrequency equal to the frequency difference between the two laser beams.The outputs of the photo-detectors 335 go to a frequency differenceprocessor 336 which measures the frequency of the beat signal, thus thefrequency difference between the two lasers. Two detectors are necessaryto determine the sign of the frequency difference.

The RIM servo loop 324 compensates for the offset between the actualresonance frequency and the frequency at which the demodulation outputsignal equals zero (the intensity modulation bias error). The intensitymodulator 326 corrects for this difference by controlling intensityvariations occurring at frequencies around the modulation frequency. Theintensity modulator 326 is controlled by a negative feedback signalreceived at its control port from the demodulating feedback servo 332.In one embodiment, the tap coupler 328 takes a small portion of light,typically 5% to 10% of the overall beam, and re-directs the light to theservo loop photo detector 330. The servo loop photo detector 330 outputsa voltage signal proportional to the amplitude of the intensitymodulation in the light signal.

In one embodiment, the demodulating feedback servo 332 digitizes thevoltage signal, providing the digitized samples to independent in-phaseand quadrature-phase feedback control loops. By demodulating the errorsamples at the resonance detection modulation frequency, in-phase andquadrature-phase error signals are derived representing the in-phase andquadrature-phase components of the residual intensity modulation presentin the light beam. The in-phase and quadrature-phase error signals areused to control the amplitude of respective in-phase andquadrature-phase reference sine-waves, which are summed together toproduce a sinusoidal feedback control signal to intensity modulator 326.By controlling the residual intensity modulation as it appears at theresonance detection modulation frequency, a relatively slow intensitymodulator may be employed for intensity modulator 326, avoiding the needfor an expensive high speed intensity modulator.

FIG. 4 is a flow chart illustrating a method of one embodiment of thepresent invention for canceling residual intensity modulation in an RFOGlight beam. The method begins at 410 with generating in-phase intensitymodulation error signal by demodulating digital samples of a light beamusing a first sinusoidal reference generated at the resonance detectionmodulation frequency.

In one embodiment, the in-phase intensity modulation error signal isgenerated by a first loop of a demodulating feedback servo such asdescribed with respect to FIGS. 1 and 2.

The method proceeds to 420 with generating a quadrature-phase intensitymodulation error signal by demodulating the digital samples of the lightbeam using a second sinusoidal reference generated at the resonancedetection modulation frequency, wherein the second sinusoidal referenceis out-of-phase from the first sinusoidal reference by 90 degrees. Inone embodiment, the quadrature-phase intensity modulation error signalis generated by a second loop of the demodulating feedback servo such asdescribed with respect to FIGS. 1 and 2.

The method proceeds to 430 with producing a sinusoidal feedback controlsignal from the in-phase intensity modulation error signal and thequadrature-phase intensity modulation error signal. As described above,by adjusting the amplitude and phase of the sinusoidal feedback signalto drive the in-phase intensity modulation error signal and thequadrature-phase intensity modulation error signal to zero, the residualintensity modulation in the RFOG light beam can be controlled andcanceled from the light beam.

Accordingly, the method proceeds to 440 with driving an intensitymodulator with the sinusoidal feedback control signal, wherein theintensity modulator cancels residual intensity modulation from the lightbeam. In one embodiment, the sinusoidal feedback control signal isproduced by generating a first control signal that is a function of thefirst sinusoidal reference multiplied by the in-phase intensitymodulation error signal, and generating a second control signal that isa function of the second sinusoidal reference multiplied by thequadrature-phase intensity modulation error signal. Then the firstcontrol signal and the second control signal are summed to produce thesinusoidal feedback control signal. The resulting amplitude and phase ofthe sinusoidal feedback control signal will be a function of theamplitudes of the first control signal and the second control signal.

Several means are available to implement the systems and methods of thecurrent invention as discussed in this specification. These meansinclude, but are not limited to, digital computer systems,microprocessors, general purpose computers, programmable controllers andfield programmable gate arrays (FPGAs) or application-specificintegrated circuits (ASICs). Therefore other embodiments of the presentinvention are program instructions resident on computer readable mediawhich when implemented by such means enable them to implementembodiments of the present invention. Computer readable media includeany form of a physical computer memory storage device. Examples of sucha physical computer memory device include, but is not limited to, punchcards, magnetic disks or tapes, optical data storage system, flash readonly memory (ROM), non-volatile ROM, programmable ROM (PROM),erasable-programmable ROM (E-PROM), random access memory (RAM), or anyother form of permanent, semi-permanent, or temporary memory storagesystem or device. Program instructions include, but are not limited tocomputer-executable instructions executed by computer system processorsand hardware description languages such as Very High Speed IntegratedCircuit (VHSIC) Hardware Description Language (VHDL).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

1. A resonant fiber optic gyroscope (RFOG) having a residual intensitymodulation (RIM) controller, the controller comprising: an intensitymodulator optically coupled to receive a light beam from a laser sourcemodulated at a resonance detection modulation frequency; an optical tapdevice optically coupled to the intensity modulator; and a feedbackservo coupled to the optical tap device and the intensity modulator, thedemodulating feedback servo generating a sinusoidal feedback signal tothe intensity modulator; wherein the feedback servo adjusts an amplitudeand phase of the sinusoidal feedback signal provided to intensitymodulator based on a residual intensity modulation detected by thedemodulating feedback servo.
 2. The controller of claim 1, wherein theintensity modulator applies a modulation to the light beam to cancel atleast part of the residual intensity modulation based on the sinusoidalfeedback signal.
 3. The controller of claim 1, wherein the sinusoidalfeedback signal has a frequency at the resonance detection modulationfrequency.
 4. The controller of claim 1, further comprising ananalog-to-digital converter coupled to the optical tap device; whereinthe analog-to-digital converter generates digital samples of the lightbeam as received at the optical tap device, wherein the digital samplesare provided to the feedback servo.
 5. The controller of claim 1,wherein the feedback servo comprises: an in-phase demodulator thatdemodulates the digital samples using a first reference frequency at theresonance detection modulation frequency to produce a first error signalthat represents an in-phase component of the intensity modulation; aquadrature-phase demodulator that demodulates the digital samples usinga second reference frequency at the resonance detection modulationfrequency to produce a second error signal that represents aquadrature-phase component of the intensity modulation, wherein thesecond reference frequency is 90 degrees out of phase from the firstreference frequency; wherein the demodulating feedback servo adjusts theamplitude and phase of the sinusoidal feedback signal based on drivingboth the first error signal and the second error signal to zero.
 6. Thecontroller of claim 5, further comprising a summing amplifier; whereinthe feedback servo generates a first analog error signal based onmultiplying the first reference frequency by the first error signal;wherein the feedback servo generates a second analog error signal basedon multiplying the second reference frequency by the second errorsignal; and wherein the summing amplifier sums the first analog errorsignal with the second analog error signal to produce the sinusoidalfeedback signal.
 7. The controller of claim 5, the feedback servofurther comprising: a first accumulator coupled to the in-phasedemodulator, wherein the first accumulator integrates an output of theof the in-phase demodulator to produce the first error signal; and asecond accumulator coupled to the quadrature-phase demodulator, whereinthe second accumulator integrates an output of the quadrature-phasedemodulator to produce the second error signal.
 8. A residual intensitymodulation (RIM) servo device for a resonant fiber optic gyroscope(RFOG), the device comprising: a first control loop comprising anin-phase demodulator that receives digital data samples of a light beam,the first control loop generating a first error signal that is afunction of an in-phase component of residual intensity modulation inthe light beam; a second control loop comprising a quadrature-phasedemodulator that receives the digital data samples of the light beam,the second control loop generating a second error signal that is afunction of a quadrature-phase component of residual intensitymodulation in the light beam; an amplifier coupled to the first controlloop and the second control loop, the amplifier generating a sinusoidalfeedback signal that drives an intensity modulator that modulates thelight beam, wherein the amplitude and phase of the sinusoidal feedbacksignal are controlled with the first error signal and the second errorsignal.
 9. The device of claim 8, wherein the amplitude and phase of thesinusoidal feedback signal adjust the intensity modulator to drive boththe first error signal and the second error signal to zero.
 10. Thedevice of claim 8, wherein the intensity modulator applies a modulationto the light beam to cancel at least part of the residual intensitymodulation based on the sinusoidal feedback signal.
 11. The device ofclaim 8, wherein the sinusoidal feedback signal has a frequency at theresonance detection modulation frequency.
 12. The device of claim 8,wherein the first control loop further comprises a first accumulatorcoupled to the in-phase demodulator, wherein the first accumulatorintegrates an output of the in-phase demodulator to produce the firsterror signal; and wherein the second control loop further comprises asecond accumulator coupled to the quadrature-phase demodulator, whereinthe second accumulator integrates an output of the quadrature-phasedemodulator to produce the second error signal.
 13. The device of claim8, further comprising: an analog-to-digital converter coupled to anoptical tap device receiving the light beam; wherein theanalog-to-digital converter generates the digital samples of the lightbeam, wherein the digital samples are provided to the a first controlloop and the second control loop.
 14. The device of claim 8, furthercomprising: a first signal generator that produces a first referencesignal at an operating modulation frequency of the RFOG, wherein thein-phase demodulator demodulates the digital data samples using thefirst reference signal; and a second signal generator that produces asecond reference signal at an operating modulation frequency of theRFOG, wherein the quadrature-phase demodulator demodulates the digitaldata samples using the second reference signal, and wherein the firstreference signal and the second reference signal are out of phase by 90degrees.
 15. The device of claim 8, further comprising: a tap deviceoptically coupled to the intensity modulator, wherein the tap deviceprovides a signal representing the light beam to the first control loopand the second control loop.
 16. A method for controlling intensitymodulation in a resonating fiber optic gyroscope (RFOG), the methodcomprising: generating in-phase intensity modulation error signal bydemodulating digital samples of a light beam using a first sinusoidalreference generated at a resonance detection modulation frequency;generating quadrature-phase intensity modulation error signal bydemodulating digital samples of the light beam using a second sinusoidalreference generated at the resonance detection modulation frequency,wherein the second sinusoidal reference is out- of-phase from the firstsinusoidal reference by 90 degrees; producing a sinusoidal feedbackcontrol signal from the in-phase intensity modulation error signal andthe quadrature-phase intensity modulation error signal; and driving anintensity modulator with the sinusoidal feedback control signal, whereinthe intensity modulator cancels residual intensity modulation from thelight beam.
 17. The method of claim 16, wherein producing the sinusoidalfeedback control signal further comprises adjusting an amplitude andphase of the sinusoidal feedback signal to drive the in-phase intensitymodulation error signal and the quadrature-phase intensity modulationerror signal to zero.
 18. The method of claim 16, further comprising:generating a first control signal that is a function of the firstsinusoidal reference multiplied by the in-phase intensity modulationerror signal; generating a second control signal that is a function ofthe second sinusoidal reference multiplied by the quadrature-phaseintensity modulation error signal; producing the sinusoidal feedbackcontrol signal by summing the first control signal and the secondcontrol signal.
 19. The method of claim 16, further comprising:generating digital samples of the light beam at a tap device thatreceives the light beam from the intensity modulator.
 20. The method ofclaim 16, wherein the first error signal is proportional to an in-phasecomponent of the residual intensity modulation; and wherein the seconderror signal is proportional to a quadrature-phase component of theresidual intensity modulation.